The invention relates to an apparatus for the measurement or machining of an object, which apparatus is provided with positioning means for positioning a part of the apparatus that is to be positioned relative to a frame of the apparatus, which frame of the apparatus includes a metrology frame which is provided at least partly with a thermal shield.
An apparatus of this kind is known from a publication by K. H. Breyer and H. G. Pressel in xe2x80x9cProgress in Precision Engineeringxe2x80x9d, pp. 56-76, entitled xe2x80x9cPaving the Way to Thermally Stable Coordinate Measuring Machinesxe2x80x9d, Springer Verlag, New York. Precision measuring apparatus can be used, for example, in component mounting machines, in wafer steppers for the manufacture of integrated circuits, in printers or in coordinate measuring machines for determining the shape of an object to be measured. Generally speaking, temperature fluctuations, that is, fluctuations in time as well as in space, constitute the major source of positioning and/or measuring uncertainty in such apparatus.
In order to realize apparatus exhibiting a low susceptibility to thermal disturbances, it would be feasible to manufacture such apparatus, or at least the parts thereof which are decisive in respect of the precision (the metrology frame), of a material having a (very) low coefficient of thermal expansion. Such a material, whose coefficient of expansion may be a factor of 1000 smaller than that of customary structural metals, is known, for example, as Zerodur(trademark). This material, however, has the drawback that it is much more expensive than customary structural metals and that it is also difficult to carry out machining operations on this material.
Another approach to solving the problem of thermal disturbances in measuring apparatus is to provide thermal insulation for the entire measuring apparatus or for the parts of the apparatus which have a comparatively strong effect on the measuring precision of the apparatus. The cited publication describes (notably on page 74) a precision measuring apparatus which is provided with a granite table for accommodating the object to be measured. The known apparatus is also provided with a metrology frame which includes positioning means in the form of bridge-like carriers which are intended to displace a part of the apparatus, that is, a part in the form of a measuring probe, in the x direction, the y direction and the z direction. Parts of these bridge-like carriers and a part of the granite table are provided with an insulating layer of polyurethane foam. This insulating layer may be considered to form a thermal shield.
In the context of the present invention a metrology frame for an apparatus for measuring or machining an object is to be understood to mean that part of the frame of the apparatus which determines the information concerning the position of the part of the apparatus to be positioned relative to the frame of the apparatus. A part of the frame of the apparatus which contributes to the supporting or positioning of parts of the apparatus or of the object to be measured, but does not transfer said information, therefore, does not form part of the metrology frame.
Even though this known method of thermal insulation offers an improvement in comparison with a non-insulated apparatus, the known apparatus is still susceptible to a number of thermal effects. It is attempted to counteract the latter effects by using the cited materials having a low coefficient of thermal expansion for some parts.
It is an object of the invention to provide an apparatus of the kind set forth in which the disturbing thermal effects are counteracted better. To this end, the apparatus in accordance with the invention is characterized in that the thermal shield consists of at least an inner shielding layer and an outer shielding layer, which outer shielding layer has a thermal conductivity which is higher than that of the inner shielding layer.
The invention is based on the recognition of two facts. The first fact is the insight that it will never be possible to completely avoid temperature variations of the apparatus in practical circumstances. Therefore, an apparatus of this kind, usually being provided with a system for automatic data processing (a computer for control of the apparatus and for processing the measured quantities), will be arranged to carry out software corrections on the data on the basis of the temperature deviations measured at the site of the apparatus, that is, in such a manner that the actual values to be measured (that is, the dimensions of the object to be measured) are approximated better. This first insight means that such a software correction can be performed in a much more reliable manner when the deformation behavior of the parts of relevance of the apparatus can be suitably predicted. The latter is the case when the temperature of the apparatus deviates from the nominal temperature, but is practically the same for the entire apparatus. In that case only one temperature sensor is required and the software corrections, moreover, correspond to a high degree to the expected shape deviations in the metrology frame of the apparatus, so that a smaller correction uncertainty is obtained. The requirement as regards a uniformly distributed temperature deviation is met to a high degree in that on the outer side (that is, the side wherefrom the disturbing thermal influences originate) the shield is provided with a thermally suitably conductive outer layer which, because of its suitable conductivity, neutralizes any temperature gradients to a high degree. Any small temperature gradients still remaining are transferred to the metrology frame of the apparatus only after thorough attenuation by the thermally insulating inner layer, so that the temperature deviation, moreover, is very small.
The second insight on which the invention is based is that any temperature fluctuations have a disturbing effect which is greater as they have a shorter period, viewed in time, that is, when they have a comparatively high frequency. This can be understood by realizing that a sudden temperature variation outside the shield causes a temperature gradient inside the shield, because the material inside the shield does not have time to equalize the applied temperature variation by conduction; as has already been described, temperature gradients must be avoided. The latter condition is satisfied by the thermal capacity of the outer shielding layer which must first be heated before the transfer of heat to the metrology frame can take place. In this case the combination of the two shielding layers acts as a low-pass filter for temperature fluctuations.
The inner shielding layer in an advantageous embodiment of the invention is formed by atmospheric air. This embodiment offers the advantage that it is inexpensive and that there are no materials on the inner side of the layer which could contaminate the sensitive measuring zone of the apparatus. Moreover, if pressure contact is realized between the insulating air and the ambient air, the apparatus can also be readily used in a conditioned atmosphere. Its use in low pressure conditions (partial or complete vacuum) is not problematic either in that case, because the insulating air is automatically also evacuated. A further advantage of the use of air (generally speaking, the use of a gas) for the inner shielding layer over an inner shielding layer in the form of a solid material (for example, polyurethane foam) consists in that a solid material which makes contact with the metrology frame will exert a force on the metrology frame in the case of thermal deformation. Even though such forces will be small, in apparatus for precision measurements (measurements of displacements of the order of magnitude of nanometers) they can still cause inadmissible deformations in the metrology frame. Such forces do not occur when air is used. The thickness of the layer of air is dependent inter alia on the dimensions of the apparatus, and hence on the shield, but preferably is not greater than approximately 2 cm in order to avoid convection.
The outer shielding layer in another advantageous embodiment of the invention is made of metal. This material can be readily machined and constitutes a comparatively good thermal conductor. The metal is preferably aluminum. The thickness of the aluminum layer of the shield is dependent inter alia on the dimensions of the apparatus, and hence of the shield, and of the desired degree of suppression of external temperature gradients, but is preferably larger than 3 mm.
In another embodiment of the invention the outer side of the outer shielding layer is provided with a surface layer which has a radiation absorption coefficient which is smaller than that of the material of the outer shielding layer. This step is based on the recognition of the fact that much of the heat applied to such apparatus originates from radiation sources in the vicinity of the apparatus. Even when the apparatus is arranged in a shielded space, the operating staff will still radiate heat to the apparatus and in many instances other radiating light sources are also present.